Recent Clean Air Act legislation mandates conformance to emission standards for SO.sub.2 and NO.sub.x. While SO.sub.2 emissions can be controlled through flue gas desulfurization processes, the most cost effective technique to reduce NO.sub.x emissions is to limit the NO.sub.x production at the time of combustion.
The formation of NO.sub.x is highly sensitive to the combustion process. NO.sub.x can be formed by the process of thermal fixation of atmospheric nitrogen, known as thermal NO.sub.x ; and by the conversion of chemically bound nitrogen within the coal, known as fuel NO.sub.x. Through experimentation, the formation of thermal NO.sub.x has been found to be highly temperature dependent. For example, one correlation indicates that above a threshold temperature of approximately 2800.degree. F., with sufficient oxygen present the rate of formation of thermal NO.sub.x doubles every 70.degree. F. Fuel NO.sub.x does not indicate a strong temperature dependence. The conversion of nitrogen in the fuel to NO.sub.x is the preferred reaction in the presence of sufficient oxygen. For coals in the United States, the nitrogen content typically ranges form 0.6 to 1.8% by weight. These high percentages generally result in fuel NO.sub.x as the primary source of NO.sub.x emissions.
The generally accepted techniques to reduce NO.sub.x formation are to reduce peak firing temperatures through the spreading of the flame and to reduce the available oxygen at the primary combustion sites. Attempts to spread the flame and reduce oxygen can have severe consequences, however, such as an increase in the amount of unburned carbon in the ash; an increase in the amount of CO emissions; increased difficulty in positioning flame scanners, thereby preventing the scanners from properly observing the flame; a reducing environment within the furnace, which promotes the corrosion of boiler components; a change in the fouling characteristics of the furnace, possibly resulting in slag formation, making it more difficult to properly clean the surfaces; and a reduction in plant performance through lower steam generation and/or higher flue gas losses.
Other combustion techniques for suppressing the generation of NO.sub.x are two-staged combustion, flue gas recirculation, reduced excess air, and sub-stoichiometric combustion. Recently, some power plants have been upgraded and retrofitted with new combustion hardware such as low NO.sub.x burners, increased cooling area of the furnace and overfire air to help reduce the levels of NO.sub.x emissions; however, some of the same serious consequences discussed above have resulted. The potential severity of these consequences on the efficiency and availability of the unit mandate that the changes undertaken to reduce NO.sub.x properly weigh these effects.
Emissions data from actual coal fired power plant testing has shown that NO.sub.x formation is strongly influenced by controllable parameters including coal flow, burners in service, inlet air temperatures, inlet air flow patterns, air staging, firing patterns, excess air levels, flue gas recirculation and others. This data indicates that the interactions leading to NO.sub.x production are complex, and that achieving the lowest possible NO.sub.x production levels without undue loss of performance or stress on equipment is complex.